(a) Field of the Invention
The present invention relates generally to data communication and, more particularly, to a method and apparatus for communication of data using a spectrally efficient modulation format.
(b) Background of the Invention
Many different modulation formats have been used for data communication, but such modulation formats typically involve communication of one, or perhaps two, symbols (each representing one or more binary digits or bits) during each signaling interval. For example, one common modulation format used for data communication is binary phase shift key modulation (also known as BPSK modulation). BPSK modulation entails modulating the phase of a single carrier signal such that the modulated carrier signal has a first predetermined phase when a one-valued bit is transmitted and a second, different predetermined phase when a zero-valued bit is transmitted. Clearly, then, when using BPSK, exactly one pulse (representing a single bit) is communicated in each signaling interval, and no possibility exists for interference between the pulse representing a first bit communicated in one signaling interval and a different pulse representing a second, different bit communicated in a different signaling interval, whether adjacent in time or otherwise.
Another modulation format used to improve on the spectral efficiency of BPSK by transmitting more than one bit in each signaling interval is quadrature phase shift keying or QPSK modulation. QPSK modulation involves transmission of two bits of information during a single signaling interval. The increase in spectral efficiency of QPSK relative to BPSK is due to the resolvability of the QPSK communication carrier signal into two orthogonal (quadrature) components. Specifically, BPSK-type modulation is implemented on each of the xe2x80x9cin-phasexe2x80x9d and xe2x80x9cquadraturexe2x80x9d components of the QPSK carrier, each BPSK-type modulation using the full bandwidth of the QPSK signal. Because the in-phase and quadrature components of a carrier signal are orthogonal to one another, there is, once again, no possibility for interference between data pulses modulated onto each of the carrier components during a signaling interval, nor any possibility for interference between pulses modulated onto a carrier component in one signaling interval and other pulses modulated onto a carrier component in other signaling intervals.
These modulation formats and many others are well-known in the art of data communication. Another modulation format is described in detail in U.S. Pat. No. 4,680,777, issued Jul. 14, 1987, to Debabrata Saha and assigned to the University of Michigan, and further in Quadrature-Quadrature Phase-Shift Keying, an article by Debabrata Saha and Theodore G. Birdsall, appearing in IEEE Transactions on Communications, Vol. 37, No. 5, May 1989. Quadrature-Quadrature Phase-Shift Keying (also known as Q2PSK modulation) is similar to QPSK modulation but entails communicating two bits of information on each quadrature component of a carrier signal during a single signaling interval. More particularly, Q2PSK modulation encodes a bit of data on each of two, mutually orthogonal, pulses on each of the two quadrature components of a carrier signal. In that way, four bits of information can be communicated in a single signaling interval. Further, because the quadrature components of the carrier are orthogonal to one another, and the data shaping pulses modulated on to the quadrature components are also mutually orthogonal, Q2PSK modulation would seem to offer twice the spectral efficiency of QPSK modulation with substantially the same per-bit energy requirement as QPSK.
This is not the case, however, because Q2PSK modulation employs pulses that are time-limited to a single signaling interval to avoid inter-symbol interference (ISI), and that are further mutually orthogonal to avoid cross-symbol interference (CSI). These restrictions on the number of pulses transmitted per signaling interval and on the shape of those pulses are inherent in Q2PSK modulation and operate to reduce the level of spectral efficiency that can be achieved by such modulation.
Similarly, BPSK and QPSK modulation, as well as many other well-known modulation formats, also mandate the use of only non-interfering pulses to ensure that the modulated signal can be demodulated by a receiver. As will be appreciated by those of ordinary skill in the art, this restriction to non-interfering pulses inherently and undesirably limits the level of spectral efficiency that can be attained by such modulation schemes. Further, as will also be recognized by those of ordinary skill in the art, the power required for data transmission should be minimized as much as possible for obvious reasons of cost and energy conservation. However, signals modulated by prior modulation schemes having multiple pulses per signaling interval cannot be demodulated with acceptable bit-error rates unless the signal power is elevated to an unacceptably high level.
The modulation method of the present invention employs, in one aspect, transmission of a modulation signal comprising simultaneous, interfering pulses to a receiver which is capable of demodulating the modulated signal and compensating for the interference to thereby recover the transmitted pulses and the underlying data signals therefrom. According to a different aspect of the modulation scheme of the present invention, three or more pulses may be transmitted simultaneously, and demodulated, regardless of the degree, if any, to which those pulses interfere with one another in time or frequency.
According to one aspect of the present invention, a modulator modulates at least two data signals, each comprising a set of digital values. The modulator comprises developing means for developing, for each data signal, a pulse of a predetermined shape, and combining means for combining the pulses and the data signals to form a combined signal. The combined signal includes signal components based on the pulses and on the digital values of the data signals, wherein at least two of the signal components of the combined signal overlap in time and in frequency.
In one embodiment, the developing means comprises a filter for each pulse, wherein the filter for a pulse is matched to the predetermined shape of that pulse. In that embodiment, each filter produces a filter output corresponding to one of the data signals. Further, the combining means combines filter outputs of a plurality of the filters to produce the combined signal.
According to another aspect of the invention, the modulator further comprises modulating means for modulating the combined signal onto a carrier frequency to thereby form a modulated signal.
A demodulator may be provided, comprising a receiver for receiving the modulated signal and demodulating means coupled to the receiver for demodulating the modulated signal and determining a digital value of at least one of the data signals for which signal components of the combined signal overlap in time and in frequency.
In addition, the carrier frequency may comprise first and second carrier components which are substantially orthogonal to one another, and the modulating means may modulate respective first and second portions of the combined signal onto each of the first and second carrier components.
In a particular embodiment, the developing means may develop a pulse of a predetermined shape for each of at least four data signals, and the combining means may form each of the first and second portions of the combined signal based on at least two of the pulses and on the digital values of at least two of the data signals.
The modulator of the present invention may be provided in combination with a demodulator which includes a receiver for receiving the modulated signal and demodulating means coupled to the receiver for demodulating the modulated signal and determining a digital value of at least one of the data signals based on which signal components of the combined signal overlap in time and in frequency. The demodulating means preferably determines all of the digital values, and may comprise means for removing the carrier signal from the modulated signal to thereby recover the combined signal. Further, the demodulating means may also comprise a filter matched to one of the pulses or a filter corresponding to each pulse, wherein each filter is matched to the pulse to which it corresponds.
Still further, the demodulating means may also comprise a sampler coupled to each of the filters as well as a maximum likelihood sequence estimator for determining the digital values.
Also according to the present invention, a demodulator comprises receiving means for receiving a modulated signal comprising at least two signal components overlapping in time and in frequency, wherein each component contains a stream of digital values; and demodulating means coupled to the receiving means for demodulating the modulated signal and determining digital values of at least one of the signal components. The demodulating means preferably comprises a maximum likelihood sequence estimation equalizer and preferably determines all of the digital values.
According to another aspect of the invention, a modulator is provided for modulating, during a series of signaling intervals, at least three data signals, each including a digital value for each signaling interval. The modulator includes developing means for developing, for each data signal, a pulse of a predetermined shape, and combining means for combining the pulses and the data signals to form a single combined signal including, in each signaling interval, signal components based on the pulses and on a digital value from each data signal.
According to yet another aspect of the invention, two, three, four or more digital values are concurrently communicated in each of a series of signaling intervals. In a specific example where two values are communicated, first and second pulses having predetermined, different shapes are first developed. A combined signal is then formed, the combined signal comprising, for each signaling interval, signal components based on the first and second pulses and the first and second digital values for that signaling interval. At least two of the signal components of the combined signal overlap in time and in frequency during a signaling interval. The combined signal is then transmitted to a receiver, where the combined signal is demodulated to determine the first and second digital values for each signaling interval.
In another specific example where three values are communicated, first, second, and third digital values are concurrently communicated in each of a series of signaling intervals. In accordance with this method, first, second, and third pulses having predetermined, different shapes are developed. A combined signal is then formed, the combined signal comprising, for each signaling interval, signal components based on the first, second, and third pulses and the first, second, and third digital values for that signaling interval. The combined signal is transmitted to a receiver and is demodulated at the receiver to determine the first, second, and third digital values for each signaling interval. Of course, one or more additional digital values can be communicated via the same combined signal on a carrier component that is orthogonal to the carrier component on which the first, second, and third digital values are communicated. Such additional digital values can be modulated either in accordance with the present invention or in conventional fashion.